How long to charge a human by USB?

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Question

If a human could be fed with energy via USB ports rather than food (like the synths in Channel Four's 'Humans) how long would it take to fully charge an average human with, say, 2000 calories of energy?

Answer

Chris Smith plugged in to answer this question...

Chris - Those 2000 Calories are "big" calories because an average person uses between 2000, 2500 Calories a day. So I thought this is quite interesting. With USB 3 you can draw up to 3 amps. Given that it's at 5 volts, that means that the rate of energy delivery from a USB is a maximum of 15 watts - in other words, 15 Joules per second. Now, one "big" Calorie - like is in food - that's equivalent to 4200 joules. If you want someone to consume 2000 big Calories in a day, which is your average, that's 2000 times 4200 joules. That's about 8 million joules. To work out how long it will take in seconds to get that much energy into someone with a USB port then you have to divide 8 million by 4200. So, that's about 53000. In other words, it takes you 6 days to get one day's worth of energy - equivalent to one day's food ration - into you. Probably not going to be a very sustainable means of nutrition unless we can up the wattage; a person runs at about 2 watts per kilo. So, on average, a 50 or 60-kilo person is running at about 100-watt. Therefore, we've got to find a way of delivering energy at that sort of rate. I think I'll stick to food. Food is nicer.

Ewen - Why don't you use more plugs, you could have several plugs to plug one person...?